1. Field of the Invention
This invention relates to a pneumatic tire for motorcycle, and more particularly to an improvement of a pneumatic rubber tire for motorcycle which can realize effective absorption and mitigation of external disturbances transferred from road surface during the running of the motorcycle.
2. Description of the Prior Art
In a motorcycle tire, there has hitherto been known an example where a rubber layer covering an outer periphery of a toroidal carcass is composed of a tread rubber for a crown portion of the carcass and a side rubber for a sidewall portion. Another example is that where the rubber layer is composed of a single rubber extending from the crown portion to the sidewall portion.
In the former example, rubber having excellent wear resistance, wet skid resistance and cornering stability is used as the tread rubber and rubber having an excellent flex resistance is used as the side rubber. On the other hand, the latter example uses rubber having such compromise properties that the properties required for the tread rubber and the side rubber are sacrificed to a certain extent. In any case, these tires do not exhibit these problem at an initial use stage, but the following troubles are caused as wear proceeds, i.e. on and after middle wearing stage:
(1) In the tire for front wheel, a so-called shimmy phenomenon is produced to cause vibration of handle bars; and PA1 (2) In the tire for rear wheel, a so-called wobble phenomenon is produced to cause vibration of a vehicle body. PA1 said tread having a laminate structure inclusive of a base rubber having a storage modulus (E') of 50-90 kg/cm.sup.2 and a ratio of loss tangent to storage modulus (tan .delta./E') of 2.5.times.10.sup.-3 -5.times.10.sup.-3 and a cap rubber having a storage modulus at least equal to that of the base rubber, preferably higher by 1.3-2.5 times than that of the base rubber; and PA1 said base and cap rubbers being arranged as one body along the carcass between the sidewalls up to positions corresponding to the maximum width of the tire.
As a result of detail investigations on the cause of these phenomena, it has been confirmed that the above phenomena result from the fact that as the rubber layer of the tread is worn and is on and after the middle wearing stage, the shearing rigidity of the tread in a direction parallel to the rotational axis of tire rises due to the wearing and finally the function of the tread for absorbing and mitigating changes transferred from road surface or external disturbances is degraded.
Apart from the investigation on the cause, it has hitherto been attempted to make a rubber gauge between the adjacent plies of the carcass thicker as a countermeasure on the above problem. By this countermeasure, the trouble is solved to a certain extent in the straight running of the motorcycle, but the trouble is still left in cornering, so that the above countermeasure can hardly be said to provide sufficient solution on the troubles.
Recently, the motorcycles have remarkable trends to make the size larger and to use at higher speed, so that the aforementioned troubles actually come into open as a serious problem in view of the safety.